FIG. 1 is a circuit diagram illustrating a configuration of a multiple-output switching power source apparatus according to a related art. In this multiple-output switching power source apparatus, an input voltage Vin is a DC voltage formed by rectifying and smoothing an AC voltage from a commercial power source. Between the input voltage Vin, a primary winding P1 of a transformer T1 and a switching element Q1 made of, for example, a MOSFET are connected in series. A control circuit 12-1 controls ON/OFF of the switching element Q1.
On the secondary side of the transformer T1, there is a rectifying-smoothing circuit connected to a secondary winding S1 that is wound so as to generate a voltage whose phase is opposite to the phase of a voltage of the primary winding P1 of the transformer T1. The rectifying-smoothing circuit has a diode D1 and a smoothing capacitor C1, to rectify and smooth a voltage induced by the secondary winding S1 of the transformer T1 and outputs the voltage as a first output voltage Vo1 from a first output terminal.
The converter to provide the first output voltage Vo1 is generally known as a flyback converter that accumulates excitation energy in the primary winding P1 of the transformer T1 during an ON period of the switching element Q1, and after the switching element Q1 turns off, discharges energy through the diode D1 to the output. A feedback circuit 10 feeds back an error signal between the first output voltage Vo1 and a reference voltage to the control circuit 12-1 on the primary side. Based on the error signal, the control circuit 12-1 adjusts an ON width of the switching element Q1, to control the first output voltage Vo1 to a predetermined value.
A second output voltage Vo2 and a third output voltage Vo3 are provided by DC/DC converters such as step-down choppers connected to the first output terminal of the flyback converter.
The step-down choppers have switching elements Q2 and Q3 and reactors L2 and L3 those are connected in series among the first output terminal, a second output terminal, and a third output terminal. Between connection points of the switching elements Q2 and Q3 and reactors L2 and L3 and the ground, there are connected regenerative diodes D2 and D3.
Control circuits 12-2 and 12-3 control ON widths of the switching elements Q2 and Q3, to thereby control output voltages. During periods in which the switching elements Q2 and Q3 are ON, power is supplied from the first output terminal to the second and third output terminals through the reactors L2 and L3. After the switching elements turn off, energy accumulated in the reactors L2 and L3 is discharged through the regenerative diodes D2 and D3 to the second and third output terminals.
In a load zone in which current continuously passes through the reactors L2 and L3, the second and third output voltages Vo2 and Vo3 are averages of voltages applied to the reactors L2 and L3. If the switching elements Q2 and Q3 have an ON duty of Don, the output voltages Vo2 and Vo3 each is approximately “Vo1×Don”. In a light load zone in which current discontinuously passes to the reactors L2 and L3, power to be outputted corresponds to ON widths of the switching elements Q2 and Q3.